The present invention concerns a non-heat treated steel for hot forging with easy fracture splitting, which is processed by hot forging to the shape of a machine part, then split into two or more components by rupture, and used thereafter in combination to form the machine part. The invention encompasses intermediate products for the machine parts.
Machine parts such as connecting rods of automobile engines have been manufactured by the following steps: forging the material into one body of the part, necessary finishing by machining, and splitting by machine into two components, i.e. , the cap or big end and the small end/rod. Splitting by a machine necessitates cut off width and requires many man-hours due to polishing after splitting. Thus, the costs for conventional production of connecting rods have been high.
As an alternative way to produce this kind of machine parts, other than the hot forging ingot steel, combination of metal powder sintering and forging may be employed. The sintering-forging itself is a complicated process, and therefore, not useful for solving the problem of the above discussed high costs.
In order to produce the connecting rods with reduced costs, there has been proposed, in addition to the use of non-heat treated steel, to carry out fracture splitting instead of machine splitting. This procedure comprises formation of a notch or notches at the location or locations of desired splitting on the forged product by machining and applying load so as to cause fracture starting from the notch or notches (Japanese Patent Disclosures Nos.9-3589. 9-176787 and 9-178785).
As the way to provide the notches as the staring points of the fracture splitting, other than the machining, various means utilizing thermal energy source such as laser beam, electron beam, plasma arc and TIG (hereinafter represented by the phrase “processing with laser”) became to be employed. At the notches provided by the processing by laser, different from the simple physical notches, the parts on which the laser beam is irradiated structure of the steel changes by heat to martensite phase, and thus, generally becomes brittle and favorable for fracture splitting. Anyway, in case where the notches are provided by the processing by laser, situation of occurrence and progress of the fracture are different from those caused by the notches provided by machining, and therefore, it is desirable that the non-heat treated steel for hot forging is a steel suitable for the above discussed fracture.
On the other hand, because the machine parts prepared by hot forging of ingot steel of conventional non-heat treated steel for hot forging have considerable resilience with the hardness range of 20-32 HRC, which is usually observed in the machine parts, fracture splitting results in significant plastic deformation in the components on the fractured surfaces thereof (by formation of something like “share lips” observed at shock tests). It has been thus a problem that the fractured surfaces may not adhere exactly.
Generally, in the production of connecting rods, in order to improve accuracy of forging after hot forging, blanks receive cold coining or sizing. The cold coining or sizing, however, gives large residual strain to the processed material. For this reason, fracture splitting causes deformation as the result of releasing the residual strain. This is also a problem of no exact combination or adhesion at the combined fractured surfaces.